The mucous membranes are the lubricating membranes lining the internal surfaces of body cavities such as the mouth. Blood vessels running very close the surface of these membranes impart a pink color to them. Some mucous membranes have pigment within them that will prevent observation of the blood vessels underneath. Other mucous membranes that can be used for observation include the internal portion of the lower eyelid, the vulva of a female and the penis of a male. Since the color of the mucous membrane is a reflection of the local blood supply, a change in color can indicate a change in either the amount or composition of the blood flowing through the vessels.
Pale mucous membranes are caused by a decrease in the number of red blood cells flowing through the vessels. This decrease can be caused by:
Yellow mucous membranes are caused by a yellow pigment (bilirubin) and is referred to as jaundice. This pigment is not limited to the blood within the blood vessels, but actually penetrates the mucous membranes and the skin. Jaundice is caused by: